This invention relates generally to scale model racing vehicles, and in particular to radio-controlled model racing cars that are powered by miniature glow plug internal combustion engines.
Radio-controlled model racing is a popular hobby sanctioned by Radio-Operated Auto Racing, Inc., among other rule making organizations. Competition events feature model cars, model aircraft and model boats. Racing heats are generally staged on a closed-circuit race course and require each competing model to complete as many laps as possible within a specified time period, with the model completing the largest number of laps being declared the winner. Some racing events are conducted over an unimproved off-road outdoor area where the model car must be steered carefully to avoid collision with obstacles. When a collision occurs, it may be necessary to drive the model car in reverse to clear the obstacle before the race can be continued.
Each model is controlled in terms of steering, throttle and forward/reverse travel by low-power, digitally encoded radio-frequency command signals at a dedicated frequency generated by a hand-held remote control transmitter. Each model is equipped with an on-board servo and radio receiver that is tuned to the same frequency as the transmitter to cause the model to turn, increase speed, slow down and reverse direction as commanded by the operator.
There are two main categories of radio-controlled scale model vehicles, battery-powered and fuel-powered. The prime mover in a battery-powered vehicle is an electric motor, while the prime mover in a fuel-powered vehicle is an internal combustion engine. Since fuel-powered vehicles generally do not have an on-board electrical generating system, a small battery is included to provide electrical power for operating on-board radio system components. The on-board radio system components include a receiver and servo motors. Conventional battery-powered vehicles achieve reversal of the prime mover (electric motor) by reversing the polarity of the applied voltage. Conventional fuel-powered vehicles have no method for reversing the internal combustion engine, and thus are not operable in reverse.
One conventional radio-controlled scale model vehicle is equipped with an on-board battery and a DC electric motor for cranking the internal combustion engine during starting, and also for providing motive power during reverse travel operation. The internal combustion engine, which is not reversible, provides operating power for the model vehicle only during forward travel operation. The forward gear is disengaged and the engine is brought to idle under servo-control to permit transfer to the DC electric motor through a power transfer linkage and a reverse gear so that the model vehicle can be propelled by electrical power in the reverse direction.
It will be appreciated that the sequential shifting operation, which requires transition to idle speed, disengagement of the engine and engagement of the electric drive motor, imposes an undesirable time delay before the vehicle motion can be completely reversed. Additionally, if direction of travel is reversed while being operated at a high rate of speed, the gearing and/or power transfer linkage can be damaged. Accordingly, there is a need for a simple, rapid and reliable means for selectively reversing the forward driving torque produced by a prime mover, for example an internal combustion engine or inertial flywheel motor that cannot be reversed, into reverse driving torque, thus eliminating the need for an on-board battery and electric drive motor for reverse travel. Additionally, a shiftable transmission is needed for use in combination with a radio-controlled scale model vehicle in which shifting from forward to reverse is performed without damaging the transmission gear train or linkage.
According to the present invention, a radio-controlled model vehicle includes a shiftable transmission that is powered by a miniature internal combustion engine during both forward and reverse travel. The transmission includes a forward/reverse torque transfer assembly that is shiftable under the control of a servo-driven shuttle. The forward/reverse torque transfer assembly includes a shiftable clutch bell coupled to the shuttle and a centrifugal lock-out clutch that permits a gear change from forward to reverse or reverse to forward only while the prime mover (the internal combustion engine) is operating at or below idle speed. Drive train shock loading and damage to the transmission and its associated parts are avoided by preventing gear changes for any engine speed above idle.
According to one aspect of the invention, the centrifugal lock-out clutch includes a spring-loaded pawl which prevents gear change while the engine is being operated above a predetermined idle speed. For engine operation or at or below idle speed, the pawl is retracted by a bias compression spring to a non-engaging position. As the engine rpm increases above idle, the inertial force developed by centripetal acceleration overcomes the bias force of the compression spring and the pawl is extended radially outwardly for positive engagement against a torque transfer pin carried on the clutch bell, and driving torque is transmitted to the wheels.
Shifting movement of the shuttle and the clutch bell are prevented by interference contact of the inertially extended pawl against the clutch bell housing when the engine is operating at speeds above idle. Shuttle shifting and clutch engagement/disengagement are automatically enabled when the engine speed drops below idle, as the bias spring moves the pawl from the shift-blocking position into the retracted, non-interfering position, so that the clutch bell can be shifted into or away from the pawl engaging position.
According to another aspect of the invention, the centrifugal lock-out clutch includes a pair of spring-loaded friction shoes. At idle speed, the friction shoes are held in the retracted (non-interfering) position by torsion bias springs, and the clutch bell is free to either forward or reverse shift to an engagable torque transfer position over the friction shoes. As the engine rpm increases above idle, the inertial forces developed by centripetal acceleration overcome the yieldable restraining force of each torsion spring, thus extending the friction shoes radially outwardly into frictional engagement against the clutch housing and transferring driving torque to the wheels. The clutch bell cannot be disengaged and shifted from one position to the other as long as the engine rpm remains above idle.
In each embodiment, the position of the clutch bell in relation to the shoe/pawl inertial stop apparatus determines whether the clutch is permitted to engage/disengage the forward gear or the reverse gear. If the clutch bell is not positioned over a shoe or pawl and the engine is operating above idle rpm, the radial extension of the shoe/pawl blocks axial shifting movement of the shuttle and clutch bell. Consequently, for any engine speed above idle, a gear change from forward to reverse or reverse to forward is not allowed.